This invention relates to a waveform equalization circuit in a magnetic reproducing device, such as a rotary head type digital audio tape recorder, which is adapted to reproduce digital signals recorded on a magnetic recording medium.
A reproducing head used for magnetic reproduction performs differential reproduction, and it therefore theoretically has an output voltage characteristic of +6 dB/octave, however, in practice the high frequency range is damped because of various factors such as for instance high frequency loss. In order to overcome this difficulty, a waveform equalization circuit has been employed which corrects the signal level so as to reproduce the recorded signal waveform with high fidelity, taking into account the output voltage characteristic of the above-described reproducing head.
A generally applied waveform equalization circuit of this type is shown in FIG. 6.
In FIG. 6, reference numeral 1 designates an input terminal to which a signal e.sub.i is applied and which is reproduced by the reproducing head (not shown) and amplified; numeral 2 designates an output terminal through which a signal e.sub.o obtained using a predetermined waveform equalization is outputted; and 3 designates an amplifier having a gain A.sub.0, and having a non-inversion input terminal to which a signal e.sub.ip is applied, a an inversion output terminal through which a signal e.sub.in is inputted, a non-inversion output terminal through which a signal e.sub.op is outputted, and an inversion output terminal through which a signal e.sub.on is outputted. Further, in FIG. 6, reference characters C.sub.1 through C.sub.3, and R.sub.1 through R.sub.3 designate capacitors and resistors, respectively, which are required for obtaining a predetermined transfer characteristic.
In FIG. 6, transfer functions G(s) and G.sub.1 (s) through G.sub.4 (s) are determined as follows: EQU G(s)=e.sub.o /e.sub.i ( 1) EQU G.sub.1 (s)=e.sub.ip /e.sub.i ( 2) EQU G.sub.2 (s)=e.sub.in /e.sub.i ( 3) EQU G.sub.3 (s)=e.sub.op /e.sub.i ( 4) EQU G.sub.4 (s)=e.sub.on /e.sub.i ( 5)
Then, the following equations (6), (7) and (8) can be obtained: EQU G.sub.1 (s)=1/(SC.sub.2 R.sub.2 +1) (6) EQU G.sub.2 (s)=SC.sub.1 R.sub.1 /(SC.sub.1 R.sub.1 +1) (7) EQU G.sub.3 (s)=A.sub.0 (G.sub.1 (s)-G.sub.2 (s)) (8)
By inserting equations (6) and (7) in equation (8), the following equation (9) can be obtained: ##EQU1## In addition, the following equations (10) and (11) can be obtained: EQU G.sub.4 (s)=-G.sub.3 (s) (10) EQU G(s)=G.sub.4 (s)+(G.sub.3 (s)-G.sub.4 (s)).1/SC.sub.3 R.sub.3 +1 (11)
By inserting equations (9) and (10) in equation (11), the following equation (12) can be obtained: ##EQU2## In equation (12), the first and second terms are represented by F.sub.1 (s) and F.sub.2 (s), respectively, as follows: ##EQU3## In this case, F.sub.1 (s) realizes the predetermined gain characteristic, and F.sub.2 (s) controls only the phase characteristic with the amplitude maintained unchanged.
When, in FIG. 6, the values of the capacitors C.sub.1 through C.sub.3 and the resistors R.sub.1 through R.sub.3, and the gain A.sub.0 of the amplifier 3 are determined so as to conform the following equations (15) through (18), then the frequency-gain characteristic and the frequency-phase characteristic are as indicated by the solid line and the broken line in FIG. 7, respectively. EQU fc.sub.1 =1/2.pi.C.sub.2 R.sub.2 =100 KHz (15) EQU fc.sub.2 =1/2.pi.C.sub.1 R.sub.1 =10 MHz (16) EQU fc.sub.3 =1/2.pi.C.sub.3 R.sub.3 =5 MHz (17) EQU A.sub.0 =1 (18)
As shown in FIG. 7, the frequency-gain characteristic of the above-described generally applied circuit is about 0 dB in a frequency range not higher than fc.sub.1, and decreases substantially at a rate of -6 dB/oct in a frequency range of from fc.sub.1 to .sqroot.fc.sub.1 fc.sub.2. The frequency-gain characteristic is the lowest at the point of the frequency of .sqroot.fc.sub.1 fc.sub.2. Then, the frequency-gain characteristic increases substantially at a rate of +6 dB/oct in a frequency range of from .sqroot.fc.sub.1 fc.sub.2 to fc.sub.2, and is about 0 dB in a frequency range not lower than fc.sub.2. The frequency-gain characteristic is determined by the factors fc.sub.1 and fc.sub.2 only; that is, the degree of freedom in determination of the frequency-gain characteristic is small. Therefore, in the waveform equalization circuit, it is difficult to optimize the characteristic to minimize the error rate of the digital signal.
In order to transfer reproduced digital signal without distortion, it is essential that a waveform equalization circuit in a rotary head type digital audio tape recorder or the like have a transfer characteristic as shown in FIG. 8 in which the shaded parts A and B are of a point symmetry. Therefore, the frequency-gain characteristic of the waveform equalization circuit must change, for instance, with the operating characteristic of a reproducing head employed. However, in the circuit shown in FIG. 6, the gain characteristic in the high frequency range is fixed at 0 dB similarly as that in the low frequency range. Therefore, when the gain in the high frequency range is required to be other than 0 dB, the provision of a low-pass filter capable of changing the gain characteristic in the high frequency range at the rear stage of the circuit of FIG. 6 is needed. The resultant circuit is unavoidably intricate in arrangement.